There have heretofore been known a chromate treatment, a phosphate treatment, a treatment using a silane coupling agent, an anodic oxidation treatment and the like treatment for improving the adhesion between an organic film and metal materials such as a steel plate, a zinc-plated steel plate, an aluminum-plated steel plate, a zinc alloy plate, a tin-plated steel plate, an aluminum foil, an aluminum alloy plate and a magnesium alloy plate, and for joining a metal material with a metal material of the same kind or of a different kind by using an adhesive.
The metal materials obtained through the above treatment have been widely used for such applications as domestic electric appliances, building materials, vehicles, aircraft, containers and the like. Among them, the chromate treatment has been used most widely owing to its excellent corrosion resistance and intimate adhesion.
From the standpoint of the treatment methods, the chromate treatments can be roughly divided into those of the chemical type (reaction type/coated type) and those of the electrolytic type. From the standpoint of the formed films, the chromate treatments can be roughly divided into those of the type in which a trace amount of hexavalent chromium remains in the final products for utilizing the self-restoring effect to a large extent and those of the type in which the hexavalent chromium does not remain in the final products.
As for those of the type in which a trace amount of the hexavalent chromium remains in the final products, it has been pointed out that the chromium is probable to elute out into the environment such as soil after the disposal. Therefore, it is a trend to abandon the use of the chromate treatment chiefly in European countries. Besides, the chromate treatment of either type uses a treatment solution which contains the hexavalent chromium which is a harmful substance and is, hence, accompanied by various problems from the standpoint of environment. Namely, it is important to completely treat the drain water of the hexavalent chromium-containing treatment solution and exhaust gas thereof, so that they will not be discharged to the outer side. Therefore, a tremendous amount of cost is required for the facilities for treating the drain water and exhaust gas, and for the disposal treatment. Besides, the rules are becoming stringent for moving the sludge after the drain water is treated and for treating the exhaust gases. Therefore, it has been desired to develop a non-chromium surface treatment that is comparable to the conventional chromate treatment.
Metal materials for metal containers have now been subjected to the chromate treatment of the type in which the hexavalent chromium does not remain in the final products and, besides, the surfaces thereof have been coated with an organic resin. For example, the tin-plated steel plate is subjected to the cathodic electrolysis in an aqueous solution of a sodium bichromate, the steel plate is subjected to the cathodic electrolysis in an aqueous solution of fluoride-containing chromium anhydride, and an aluminum alloy is subjected to the chromium phosphate treatment followed by the coating with an organic resin.
As the non-chromium surface treatment, a chemical film containing chiefly oxides of zirconium and/titanium has been formed on the surfaces of an aluminum-containing metal material by using an acidic treatment solution that contains zirconium, titanium, a compound thereof, phosphate and fluoride and that has a pH of about 1.0 to 4.0. Depending upon the compatibility with the organic resin, further, there have been put into practical use those materials without the chemical film at all (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 131937/1977).
In recent years, from the standpoint of sanitation of the metal containers and retaining flavor of the contents, there has been widely used a pre-coated material coated with a polyester resin. However, the polyester resin permits water to permeate through more than when the traditional epoxyphenol coating material or the acrylic epoxy coating material is used. When the polyester resin is pre-coated, therefore, limitation is imposed on the contents from the standpoint of adhesion and corrosion resistance unless the chromate treatment is used. Further, when aluminum coated with the polyester resin is used as an aluminum lid, there remains a problem of insufficient adhesion despite the chromate treatment is effected.
That is, a can coated with the polyester resin, which is an example of a product obtained by working a pre-coated material, offers an advantage in that the pre-coated metal material can be used as a starting material. However, the above can is accompanied by such problems as drop of adhesion of the polyester resin at the highly worked portions, corrosion starting with the cracked portions in the polyester resin caused by shocks such as of when the can has fallen down, drop of adhesion during the retort sterilization, induction of corrosion due to ions penetrated from the components of the content despite the polyester film itself is flawless, which are different from the problems of the conventional production method of effecting the surface treatment and the post-coating after the can has been formed.
On the other hand, the can lids have heretofore been using a pre-coated material for forming the coating material as a coil coating. From the standpoint of retaining flavor of the content and sanitation, however, study has been vigorously forwarded by utilizing a pre-coated material coated with the polyester resin. In an easy-open lid coated with the polyester resin, the delamination in which the resin peels off the metal easily occurs near the score opening due to a decrease in the adhesion to the polyester resin, causing the resin to be drawn at the opening, i.e., causing a feathering phenomenon to easily take place. In the can lid immediately after the retort sterilization, in particular, the opening tends to become defective decreasing the adhesion to the resin.
From the above point of view, there have been proposed a method of forming an organic/inorganic composite film containing an organic compound comprising chiefly carbon, and a phosphorus compound, and a zirconium compound or a titanium compound (Japanese Unexamined Patent Publication (Kokai) No. 11-229156), and a method of forming a surface-treating layer comprising chiefly an inorganic material on the surface of an aluminum base material and further forming thereon an organic surface-treating layer comprising chiefly an aqueous phenol resin (Japanese Unexamined Patent Publication (Kokai) No. 2001-121648). From the standpoint of the lid material, further, there has been proposed the anodic oxidation processing (see Japanese Unexamined Patent Publications (Kokai) No. 11-91034 and 2002-266099).
There has further been proposed the treatment of aluminum material with a polyacrylic acid/zirconium compound (Japanese Unexamined Patent Publication (Kokai) No. 6-322552 and “Light Metals”, 1990, pp. 298-304).
In order to form a zirconium oxide film, further, attempts have been made to form ZrO2 relying upon the PVD or the CVD, or to form an oxide film by applying an organic zirconium compound relying upon the wet method followed by hot drying.
Moreover, study has been conducted to form a zirconium oxide powder or a film on a graphite or titanium substrate by the cathodic electrolysis in an aqueous solution containing zirconium ions (“Materials Research Society Symposium Proceedings”, 1988, Vol. 121, pp. 111-114 and “Journal of Electrochemical Society”, 1991, Vol. 138, No. 7, pp. 1939-1942).